This application claims the priority of German Application No. 19925132.0, filed Jun. 2, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a fluid friction clutch with a primary disk fastened on a driving shaft consisting of magnetically active material and a driven clutch housing surrounding the primary disk and journaled with a hub on the shaft, and with a working chamber formed between the primary disk and clutch housing into which a shear fluid can enter from a reservoir chamber according to the position of a valve plate which opens or closes a connecting opening between reservoir and working chamber and which has an armature lying in the vicinity of the shaft, which can be shifted against spring action by a magnetic force produced by a stationary electromagnetic coil arranged on the driven end of the shaft into the open or closed position.
Liquid friction clutches of this kind as known (DE 27 18 723 A1). In the known types the magnetic flux of the electromagnetic coil disposed on a fixedly held sleeve is transferred exclusively through the driven shaft to the armature of a valve plate disposed coaxially to the shaft. Since between the magnet coil and the armature to be actuated by its field the distance is great, the coil must be of correspondingly great dimensions if the use of such a design is to be possible in practice.
Liquid friction clutches of a similar kind have become known (DE 31 09 724 A1) in which an axially displaceable stem has been laid in the driven shaft, which with one end operates the valve plate and reaches at the other end into the range of an electromagnetic coil which is stationary and reaches into a cavity in the shaft. Such designs are quite complicated on account of the need to move the valve operating parts mechanically and, due to the danger of the seizing of the valve stem, tend to give trouble.
Other types of electromagnetically operated fluid friction clutches, such as those shown in U.S. Pat. No. 4,346,794, since the magnetic flux works through the driven shaft and the clutch housing, can be used only if the clutch housing consists of magnetic material.
The present invention is addressed to the problem of designing a fluid friction clutch of the kind described above such that the operation of the valve can be performed effectively with the least complication if the clutch housing and/or the primary disk do not consist of magnetic material.
In a fluid friction clutch of the kind mentioned above, this problem is solved in that the hub of the clutch housing facing the electromagnetic coil is provided with an inserted first conducting ring of magnetic material which is magnetically coupled with one end to the electromagnetic coil, and at its other end facing the working chamber adjoins with a gap a second conducting ring of magnetic material which passes through the primary disk and ends in an area in front of the armature of the valve plate.
On account of this configuration, since the driven shaft itself consists of magnetically conductive material, a closing of the magnetic lines of force is produced in a relatively simple manner, and suffices to operate the valve even if the electromagnetic coil itself is not oversized.
In a further advantageous feature of a preferred embodiment of the invention, both conductive rings can be configured in the manner of crowns with protruding points and can be embedded in the material of the clutch housing and primary disk, and especially cast therewith. This is advantageous especially when primary disk and clutch housing consist of aluminum.
In a further advantageous feature of a preferred embodiment of the invention, the first conducting ring can be with an outwardly facing flange at a short distance from a magnetic flux ring, and the second conducting ring can have on its side reaching into the working chamber an inwardly facing flange in contact with a radial wall of the primary disk; so that the magnetic flux can be guided directly by the electromagnetic coil into the area in front of the armature of the valve plate and back again from there through the shaft on which the electromagnetic coil is placed in a known manner likewise through a magnetically conductive ring.
An advantageous feature of a preferred embodiment of the invention the gap between the two conductive rings can be closed off tightly from the working chamber by a wall portion of the hub, so That the arrangement of the conductive rings will not cause the clutch housing to have leaks.
In a further advantageous feature of a preferred embodiment of the invention, the shaft can be provided at its and facing the working chamber with a projection which serves as a pole for the armature, and this projection can be provided with an end face which lies in one plane together with the inwardly directed margin of the second conductive ring. The second conducting ring and shaft therefore form a planar contact surface which is closed by an armature plate of the valve and in this manner provides such that the magnetic force can act especially well on the valve.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.